Use JSON for all test requests

Requires !2418 (merged), closes #1829 (closed)

Making the switch showed a bug in CreateElementChildren: calling it with {"children": []} did not cause an error as expected. Since the multipart format does not support lists at all, it sent nothing, so the endpoint interpreted it as a required field being missing rather than a field set to an empty list, so the bug went unnoticed.

I removed all of the format="json" we were using in the requests to force the format when the multipart format was causing us trouble, since it is now the default.

changed milestone to %Arkindex 1.6.3

requested review from @babadie

assigned to @erouchet

added 3 commits

• e8cafaf1...71ba7acf - 2 commits from branch master
• 4995d292 - Make JSON requests in unit tests

Compare with previous version

Tangent question: why do we keep rendering DRF output as HTML for humans in prod ?

merged